Forming Limit Curve (FLC) and fracture mechanism of newly developed low-carbon low-silicon TRIP steel

Abstract

The Forming-Limited Diagram (FLD) of intercritically annealed 0.11C-1.65Mn-0.62Si TRIP-assisted steel was investigated. The high FLD0 value of this new low carbon TRIP steel was indicative of a superior formability. The micro-structural changes during deformation and fracture were studied in detail. The polygonal ferrite phase was found to plastically deform first and deformed most at larger strains. Fracture was initiated by micro-voids nucleated at ferrite grain boundaries, within ferrite grains or at the interface between ferrite and the harder phases. Cracks were formed after micro-voids grew, coalesced, and expanded in one direction. When crack tips reached the bainite phase or the martensite/austenite constituent, the cracks propagated along the boundary of these phases. Cracks reaching retained austenite islands caused stress-induced martensite transformation at the crack tip. The direction of motion of the cracks also changed in this case.